The present embodiments relate to a magnetic resonance antenna arrangement for a magnetic resonance system with at least one first antenna group including a plurality of individually-controllable first antenna conductor loops.
Magnetic resonance tomography involves a technique of obtaining images of the inside of the body of a living proband. In order to obtain an image with this method, the body or the part of the body of the patient or proband under examination is to be subjected to a static basic magnetic field (B0 field) that is as homogeneous as possible. The basic magnetic field is created by a basic field magnet system of the magnetic resonance system. The basic magnetic field is overlaid during the magnetic resonance imaging with rapidly switched gradient fields for local encoding. The gradient fields are generated by gradient coils. High-frequency pulses of a defined field strength (e.g., the “B1 field”) are beamed (e.g., radiated) with high-frequency antennas into the object under examination. The nuclear resonance of the atoms in the object under examination are excited by the high-frequency pulses, such that the high-frequency pulses are deflected by an “excitation flip angle” from the position of equilibrium in parallel to the basic magnetic field. The nuclear resonances process around the direction of the basic magnetic field. The magnetic resonance signals generated thereby are received by high-frequency receive antennas. The magnetic resonance images of the object under examination are created based on the received magnetic resonance signals.
To send out the high-frequency pulses into a measurement space, in which the object under examination is located, and, if necessary, also to receive the magnetic resonance signals from the object under examination, the tomograph may have a high-frequency antenna permanently installed in the tomograph housing (e.g., a “whole-body antenna”). Typical structures for whole-body antennas are birdcage structures, transversal electromagnetic (TEM) antennas, and saddle coils.
With modern magnetic resonance systems (MR systems), for example, that operate with basic magnetic field strengths of 3 Tesla or more, the interaction of the object under examination or patient with the fields of the high-frequency antenna arrangements results in degradations in the image quality. Eddy currents that may occur in the body of the patient may be responsible for these. The degradation in the image quality takes the form of a spatial variation of the flip angle in the transmit phase or variations of the signal-to-noise ratio during receiving. In addition, with these types of high magnetic field strengths, the absorption of the transmit power of the high-frequency pulses in the object under examination (e.g., the specific absorption rate (SAR)) plays a greater part. Thus, some imaging sequences are restricted in quality by the strict limitation of the permitted power absorption. To resolve or reduce these problems, the previous usual simple circular polarized transmit antennas are no longer used, but antenna arrays are used instead. The antenna arrays include a plurality of individual antenna elements (e.g., elements controllable independently of one another), or antenna conductor loops. If a multichannel transmit system, with which the individual antenna elements or antenna conductor loops may have high-frequency pulses applied independently, is also used, in principle, the high-frequency excitation field and thus the spatial flip angle distribution may be selected in any given manner. This enables, for example, a reduction in the SAR load on the patient to also be achieved. Since with these types of systems several RF pulses are transmitted simultaneously in parallel, which then overlay each other in an intended manner, this technique is also referred to as “parallel transmission technique” (pTX), and the antenna arrays are referred to as “pTX” arrays. Such antenna arrays have been used for the receive coils in the local coils to be accommodated close to the object under examination. Thus, the signal-to-noise ratio may be improved during reception, and the imaging time may be reduced.
A major demand on the antenna arrangement for pTX arrays is a sufficient decoupling of the individual antenna elements or antenna conductor loops in order to achieve crosstalk and thus a possible mixing of the separate transmit channels. With antenna arrangements, of which the antenna elements are only disposed over the circumference of the measurement space embodied, for example, in a cylindrical shape (e.g., a “patient tunnel”), a number of practical options are known for decoupling such as, for example, an overlap of two adjacent antenna conductor loops along the circumference by a specific amount. However, sufficient decoupling of antenna elements in two directions (e.g., not only in the circumferential direction but also in the longitudinal direction (axial direction) of the patient tunnel of antenna arrangements disposed next to one another) is problematic. The methods previously known for receive antennas are unsuitable for antenna arrangements that are to be employed for transmission, since the preamplifier decouplings used in the receive area may not be applied to transmit antennas. pTX antenna arrangements having individually-controllable antenna elements or antenna conductor loops not only disposed next to one another in the circumferential direction but also in the axial direction may also be built. The individual antenna elements arranged in the circumferential direction essentially only allow a direct improvement of the excitation in a transversal plane (e.g., a plane perpendicular to the axial direction of the patient tunnel). Other planes are only able to be slightly influenced by this and, even then, only when higher power is provided, which once again is associated with greater SAR. To also be able to influence any other given planes within the measurement space, three-dimensional areas or rotated slices (e.g., in the optimum manner, independently-controllable antenna elements disposed next to one another) are thus also provided in the axial direction.